This invention relates to apparatus, such as a magnetic locator, for measuring or detecting magnetic phenomena, such as magnetic fields, objects, or disturbances, and is more particularly concerned with improved magnetic sensors which may be employed in such apparatus.
Saturable core (fluxgate) magnetic locators or gradiometers comprise at least two electrically matched field-sensing elements mounted on a non-magnetic structure such that their magnetic axes are, theoretically, precisely parallel or coaxial. The output signals of the two sensors are arranged such that they oppose each other. If the structure is oriented in any direction in a uniform magnetic field, the components of magnetic field existing at each sensor are equal, so that there is no resulting output signal from the combination of the two sensors.
If a magnetic object exists within the detection range of the instrument, the magnetic field will generally be stronger at one of the sensors than at the other sensor. As a result, the output signal of one sensor will be greater than that of the other, so a net difference signal will be produced that is indicative of the presence of the object.
For accuracy of operation, the magnetic axes of the two sensors must be precisely aligned. The precision required is of the order of three seconds of arc if the error signal due to misalignment of the magnetic axes is to be less than 1 gamma (10.sup.-.sup.5 gauss) in an ambient magnetic field of 60,000 gammas. If the magnetic axes of the two sensors are not precisely aligned, the component of the ambient magnetic field existing along the magnetic axis of one sensor will not be the same as the component of the magnetic field existing along the magnetic axis of the other sensor. The difference between the two sensor signals will not be zero, and a false signal will be obtained due to the mechanical misalignment.
In certain prior art fluxgate gradiometers alignment has been achieved by mechanical adjustment or bending of structural members, such as a tube in which the sensors are mounted. See, for example, the applicant's prior U.S. Pat. No. 3,050,679, issued Aug. 21, 1962. The bending of the tube places the tube under stress. In time, the stress may be relieved and the sensors may become misaligned, resulting in inaccuracy and the need for readjustment.
The applicant's prior U.S. Pat. No. 3,488,579, granted Jan. 6, 1970, discloses a system by which compensation for misalignment is produced electrically. While this system is capable of compensating for misalignment with high precision, its cost may be higher than is justified when such high accuracy is not required.
It has heretofore been proposed to provide bodies of easily magnetizable material adjacent to gradiometers for compensating for extraneous magnetic effects and misaligned cores. See, for example, U.S. Pat. No. 2,966,853, granted Jan. 3, 1961 to Gilfillan et al; U.S. Pat. No. 3,012,191, granted Dec. 5, 1961 to Miller et al; and U.S. Pat. No. 2,976,483, granted Mar. 21, 1961 to Moore et al. However, such schemes for compensating for extraneous effects and misaligned cores have been capable of minimal adjustment and limited flexibility, and have required the attachment of additional elements to the sensor cores themselves with commensurate complexity and susceptability to errors resulting from core stresses. The applicant's prior U.S. Pat. Nos. 3,487,459, granted Dec. 30, 1969 and 3,757,209, granted Sept. 4, 1973, disclose improved mechanically adjustable compensators which avoid such problems.
In certain other prior art gradiometers, the magnetic axes of a pair of sensors are aligned by mechanically, pivotally adjusting one or both sensors relative to a support member common to both sensors. See, for example, U.S. Pat. No. 2,620,381 to Mayes et al; U.S. Pat. No. 2,642,479 to Jones; U.S. Pat. No. 3,064,185 and No. 3,065,413 to Ferguson. The structures employed in these gradiometers to effect pivotal adjustment of the sensors, which have considerable mass, are complex and costly.